Maleic anhydride can be obtained by partial oxidation of hydrocarbons, especially benzene or C4-hydrocarbons, such as 1,3-butadiene, n-butene or n-butane. The reaction is strongly exothermic and requires sufficient removal of the heat of reaction. In general, the reaction is carried out in a tube bundle reactor with a salt circulation system or a fluidized bed. The maleic anhydride formed in the reaction is typically absorbed from the crude product mixture formed in a solvent. This absorbs not only maleic anhydride but also further components present in the crude product mixture, for example including the water formed in the oxidation. The water reacts partly with the maleic anhydride to give maleic acid, which in turn isomerizes partly to fumaric acid. Fumaric acid is a dicarboxylic acid which has very sparing solubility in water or organic solvents, forms deposits and as a result can block plant parts, for example columns, heat exchangers, pumps, pipes and the like.
There are already some proposals in the prior art for preventing such blockages caused by fumaric acid.
For instance, WO 96/029,323 describes washing the fumaric acid-comprising absorbent, after stripping out maleic anhydride, with an aqueous extractant in order thus to prevent deposits. A disadvantage of this process is the high level of complexity which is needed to mix the washing water into an industrial scale system for preparing C4-dicarboxylic acids or derivatives thereof and to separate the phases again. In addition, the unavoidable loss of valuable product and solvent causes high costs. Moreover, the additional introduction of water into the process enhances fumaric acid formation further.
DE-10 2006 024 903.8 proposes completely or partly catalytically hydrogenating the fumaric acid-comprising absorbent after stripping out maleic anhydride, and recycling it completely or partly into the absorption stage.